I mostly agree with the above, but will say that I often dislike flat tapers and prefer a very modest "bulge". It also depends a bit on the kits. Slims and Euros look fine with strait flat or tapered barrels, but when I see a Sierra with a strait barrel it just doesn't look right. Too much of a bulge looks wrong too. IMO what happens between the bushings should fuse the transitions that the kit parts start...
Cigars look great to me with a bit of a bow to them, but I don't really dislike strait like I do with the Sierras. Capped kits can go either way for me. I like the look and feel of a bit of a curve, but strait barrels can have a very classy look, and with complex segmenting, strait barrels, to me at least, increase the impression of the precision involved.